https://en.wikipedia.org/wiki/Sabarimala (This one is famous for the seasonal pilgrimage)
https://en.wikipedia.org/wiki/Vadakkunnathan_Temple (350 years old, famous for the architecture)
https://en.wikipedia.org/wiki/Guruvayur_Temple (A lot of people gets married in this temple)
https://en.wikipedia.org/wiki/Padmanabhaswamy_Temple
(Now famous for the recent discovery of huge gold stash)
https://en.wikipedia.org/wiki/Padmanabhaswamy_Temple#Temple_...
To visit these temples, you would start your trip from Thrissur district all the way down south to Trivandrum district. All of the places near to the temple has got 3 star hotels for stay and all the cities are easily accessible by train except Sabarimala. Almost everyone in Kerala can satisfactorily converse in English and all routes and signs are written in English too. Kerala is a tourist friendly state.